Linear arrays of semiconductor lasers comprised of active layer stripes of semiconductor lasers arranged one dimensionally and having a continuous wave (CW) output of about 50 W have been available. Linear arrays of semiconductor laser, for example as shown in FIG. 1, comprise from 10 to several tens of stripes with ends of widths of 100 μm to 200 μm serving as emitters arranged at fixed intervals in a plane over a total width of 1 cm.
By stacking several of these semiconductor laser linear arrays to form a two-dimensional array as shown in FIG. 2, it is possible to easily obtain an increased output. Such a two-dimensional semiconductor laser array is called a “semiconductor laser stack array” or a “semiconductor laser stack bar”. Ones with kW class outputs are available on the market. If it were possible to directly condense the laser beams of the stack array using an optical system so as to obtain a sufficiently fine spot, it is likely possible to use such laser beams for a various applications such as laser materials processing.
It may be possible to obtain from a single semiconductor laser stack array a light source comprised of line segments arranged in a two-dimensional array emitting (10 to several tens)×n number of laser beams where n is the number of stack layers. Further, a high output semiconductor laser such as a quasi-CW semiconductor laser provides a light source with a large number of emitters arranged densely, with emission beams mixing with emission beams from adjoining lasers right after emission, and with substantially consecutive linear light sources arranged corresponding to the number of stack layers.
Each stripe beam is emitted from a linear light source. The beam divergence angle has a large vertical component φ with respect to the active layer of about 40° to 50° and a small parallel component θ of about 10°. Below, the large divergence angle direction perpendicular to an active layer will be called the “fast axis”, while the small divergence angle direction parallel to an active layer will be called the “slow axis”. The width of the emission light source is a narrow one of not more than 1 μm at the fast axis side and a broad one of 100 μm to 200 μm as explained above at the slow axis side.
For example, consider a laser diode (“LD”) stack array comprised of several stacked linear arrays each comprised of 12 stripes of thicknesses of 1 μm and widths of 200 μm arranged at a pitch of 800 μm. The slow axis component of a stripe beam has a beam divergence angle of 10°, so adjoining stripe beams become superposed with each other at 3.4 mm from the emitter ends of the stripes. When placing a lens after this superposition, part of the beams become beams having angles with respect to the axis of the lens and are focused at points different from the focal point of the focusing lens, so the efficiency of the system is lowered.
Therefore, in order to collimate the beams emitted from the stripe arrays using a microcylindrical lens array, it is necessary to place a lens (focal length f1≦3.4 mm) at a close position of within 3.4 mm. If multiplying the magnification (f2/f1) determined by the combination with the focal length f2 of the condensing lens for condensing the collimate beams with the width of the stripes to find the focused spot diameter, it inevitably becomes large.
In this way, in the past, it was difficult to concentrate the emission laser beams of an LD stack bar giving a light source comprised of line segments arranged in a two-dimensional array to a small area with a high density. To utilize the LD stack array for laser processing or medical applications, which account for the major part of industrial applications for high power lasers, some special means is required for concentrating the beam energy in a narrow region.
Further, if an attempt is made to use a semiconductor laser stack array as a pumped light source of a solid-state laser (as described above), since the width of the array is about 1 cm, it may not be possible to focus beams to a single spot using an ordinary lens system and the high pumping efficiency end pumping system may not be used, so application was previously only possible for side pumping systems. On the other hand, for the end pumping system in which pumping light comes from the direction of optical axis of the solid-state laser, it is possible to obtain high efficiency single fundamental lateral mode oscillation by matching the pumping space by the semiconductor laser output beams with the mode space of the solid-state laser oscillation.
Further, matching of the pumping space can be important in a double clad fiber laser, considered an advanced form of an end pumped solid-state laser. A double clad fiber laser can also be considered a high efficient brightness raising device, but the input aperture of the pumping light is a narrow some 600 μm×240 μm, so for obtaining a higher output, a special may was previously used for concentrating the rather high level semiconductor laser beams.